The present invention relates generally to apparatuses for cleaning thin discs, such as semiconductor wafers, compact discs, flat panel displays, glass substrates, and the like (i.e., wafers). More particularly, the invention relates to megasonic cleaning of such wafers.
For fabrication of semiconductor devices, thin slices or wafers of semiconductor material require polishing by a process that applies an abrasive slurry to the wafer""s surfaces. After polishing, slurry residue is generally cleaned or scrubbed from the wafer surfaces via mechanical scrubbing devices. A similar polishing step is performed to planarize dielectric or metal films during subsequent device processing on the semiconductor wafer.
After polishing, be it during wafer or device processing, slurry residue conventionally is cleaned from wafer surfaces by submersing the wafer into a tank of sonically energized cleaning fluid, by spraying with sonically energized cleaning or rinsing fluid, by mechanically cleaning the wafer in a scrubbing device which employs brushes, such as polyvinyl acetate (PVA) brushes, or by a combination of the foregoing.
Although these conventional cleaning devices remove a substantial portion of the slurry residue which adheres to the wafer surfaces, slurry particles nonetheless remain and may produce defects during subsequent processing. Specifically, subsequent processing has been found to redistribute slurry residue from the wafer""s edges to the front of the wafer, causing defects.
A number of devices have been developed to improve wafer edge cleaning. Most of the devices are employed during a separate cleaning step following the major surface cleaning. However, a few scrubbing devices have been developed which can simultaneously scrub both the major and the edge surfaces of a wafer. One such device is shown in the side elevational view of FIG. 1. The scrubbing device 11 of FIG. 1 comprises a pair of PVA brushes 13a, 13b. Each brush comprises a plurality of raised nodules 15 across the surface thereof. The scrubber also comprises a platform 19 for supporting a wafer W, a plurality of spinning mechanisms 19a-c for spinning the wafer W, and a mechanism (not shown) for rotating the PVA brushes 13a, 13b. During scrubbing, a fluid supply mechanism F supplies fluid to both major surfaces of the wafer, dislodging particles and cleaning residue from the wafer and brushes 13a, 13b. Preferably, the pair of PVA brushes 13a, 13b are positioned to extend beyond the edge of the wafer W so as to facilitate cleaning the wafer""s edges. The illustrated system further employs a separate edge brush 21, which is driven by a separate motor (not shown) that causes the edge brush to rotate. The edge brush fits over the edge of the wafer W providing more effective wafer edge cleaning.
Although the aforementioned mechanical means address the need to clean slurry residue from the wafer""s edge, they do so at the expense of increased scrubber complexity and cost, and require frequent edge brush replacement because of excessive mechanical wear. Often, megasonic wafer cleaning within a submersion tank is preferred to scrubber type cleaning, for the foregoing cost considerations and in instances when it is desirable to alter the chemistry of the cleaning solution. The commonly-used materials for scrubbing brushes have limited chemical compatibility and cannot be used with certain Chemistries. In such instances, the need for a conventional edge scrubber following megasonic cleaning significantly increases wafer cleaning time, reduces productivity, and increases the costs of wafer processing. A drawback to megasonic cleaning, however, has been that, while the major surfaces are effectively exposed to the megasonic energy, residue at the beveled wafer edges is not as effectively removed.
Accordingly, it is an object of the present invention to provide an improved method and apparatus for megasonically cleaning wafers.
It is another object of the present invention to provide an improved apparatus for optimizing megasonic cleaning of wafers.
The foregoing and other objects of the invention are realized by the present invention which comprises a megasonic cleaning apparatus having at least one reflector positioned to collect otherwise wasted cleaning energy and redirect that energy to one or a plurality of points on the wafer""s edge.
A first embodiment of the invention comprises a reflector, preferably a paraboloid reflector, which is shaped to focus all collected energy to a single point of the wafer""s edge. Energy which has been generated by a transducer and which passes the wafer will be collected by the reflector and redirected to a single point on the wafer""s edge. A plurality of such single focal point reflectors may be employed.
Another embodiment of the invention comprises a complex parabolic reflector which is shaped to provide focal points which vary along the length of the parabolic reflector, such that energy striking the reflector at different points along the reflector length will be directed to a plurality of different points along the wafer""s edge. The larger the edge surface area along which the parabolic reflector is focused, the longer the edge cleaning duty cycle experienced by the inventive cleaning apparatus. Accordingly a longer reflector is preferred to increase cleaning efficiency and throughput. Most preferably a reflector is configured to provide focal points along half of the wafer""s circumference.
Yet another embodiment comprises a simple parabolic reflector which is provided to focus on a cord along the wafer""s surface, effectively focusing cleaning energy on two points along the wafer""s edge at any given time.
Other objects, features and advantages of the present invention will become more fully apparent from the following detailed description of the preferred embodiments, the appended claims, and the accompanying drawings.